1. Field of the Invention
This invention relates to downhole tools for use in a well, and particularly to those tools utilizing a bypass to allow well fluids located below the tool to bypass the main fluid passages of the tool as the tool is lowered into a well.
2. Description of the Prior Art
During the course of drilling an oil well, one operation which is often performed is to lower a testing string into the well to test the production capabilities of the hydrocarbon-producing underground formations intersected by the well. This testing is accomplished by lowering a string of pipe, commonly referred to as a drill pipe, into the well with a formation tester valve attached to the lower end of the string of pipe and oriented in a closed position, and with a packer attached below the formation tester valve. This string of the pipe with the attached testing equipment is generally referred to as a well test string.
Normally when this formation tester valve is being lowered into a well, the tester valve is maintained in its closed position so that the pipe string above the tester valve will contain only atmospheric pressure, thus allowing the underground formation to be tested by opening the tester valve which allows any formation fluid from the underground formation to immediately flow into the pipe string and up to the surface.
Normally, when such a formation tester valve is being lowered into the well, it will be in the closed position as just described. It is generally desirable when lowering such a closed pipe string into a well to provide a bypass arrangement around the closed flow passage of the formation tester valve to permit well fluids from below the tester valve, and particularly from below the packer which is run immediately ahead of the tester valve, to flow around the closed well tester valve.
In the absence of such a bypass, the packer located below the tester valve creates a piston effect because the packer fits rather closely within the well. This piston effect impedes the rate at which the pipe can be lowered into the well and also can damage the mud filter cake on the well bore.
One example of such a bypass is seen in U.S. Pat. No. 2,740,479 to Schwegman. The Schwegman device provides a bypass which allows fluid from below the formation tester valve to flow upward through the packer mandrel and through the lower end of the tester valve, then outward through a bypass port 34 so that it may then flow upward in the annulus between the tester valve and the well bore in order to bypass the piston effect of the large packer located below the tester valve.